At represent, liquid crystal compounds, with the application field thereof being expanded more and more widely, may be applied to various types of display devices, optical devices, sensors and so on. There are various kinds of liquid crystal compounds applied to the field of display, wherein a nematic liquid crystal compound is applied the most widely. The nematic liquid crystal has been applied to passive TN and STN matrix displays and systems with TFT active matrix.
For the application field of TFT-LCD (Thin-Film Transistor Technology), in recent years, although the market has been very huge and the technology has been gradually matured, people also have constantly improving requirement to the display technology, especially to aspects of achieving fast response and reducing a driving voltage to reduce a power consumption. As one of the important optoelectronic materials of the liquid crystal display, a liquid crystal material plays an important role in improving the performance of the liquid crystal display.
As a liquid crystal material, it is required of good chemical and thermal stability, as well as stability to an electric field and electromagnetic radiation. Moreover, as a liquid crystal material for the TFT-LCD (Thin-Film Transistor Technology), it is not only required of the stabilities mentioned above, but also required of wide nematic phase temperature range, appropriate birefringence anisotropy, very high resistivity, good anti-ultraviolet performance, high charge retention rate, low vapor pressure and other performances.
For eliminating ghosting and trailing of a display screen in the dynamic picture display application, the liquid crystal is required of a very fast response speed, so, the liquid crystal is required of a lower rotary viscosity γ1. In addition, for portable devices, it is hope that the driving voltage of the liquid crystal is as low as possible, in order to reduce the energy consumption; and for displays used as TV and so on, the driving voltage of the liquid crystal is not required to be as low as that mentioned above.
The viscosity of the liquid crystal compounds, especially the rotary viscosity γ1, directly influences the response time of the liquid crystal after being powered up, and whether a rise time (ton) or a fall time (toff) is directly proportional to the rotary viscosity γ1 of the liquid crystal; due to being related to a liquid crystal cell and the driving voltage, the rise time (ton) may be adjusted through a method for increasing the driving voltage and by reducing the thickness of the liquid crystal cell; and due to being unrelated to the driving voltage but related to elastic constant of the liquid crystal and the thickness of the liquid crystal cell, the fall time (toff) is reduced by reducing the thickness of the cell. However, modes of motion of liquid crystal molecules are different under different display modes, the three modes such as TN, IPS and VA are inversely proportional to average elastic constant K, twist elastic constant and bend elastic constant respectively.
In accordance with a continuum theory of the liquid crystal, various liquid crystals will ‘rebound’ back to an original shape through an intermolecular interaction after being deformed under the action of an external force (an electric field and a magnetic field). Similarly, the liquid crystals also form ‘viscosity’ due to the intermolecular interaction. Small changes of the liquid crystal molecules will cause significant changes on the conventional parameter performance of the liquid crystal, some of these changes have a certain regularity, and some are less likely to find the regularity, which will have obvious influences on the intermolecular interaction, these influences are very delicate, and a perfect theoretical explanation has so far not formed yet.
The viscosity of the liquid crystal is related to the liquid crystal molecular structure, the study on the relationship between the viscosity and the liquid crystal molecular structure of a liquid crystal system formed by different liquid crystal molecules is one of the important tasks of liquid crystal formula engineers.